The object of DE 40 13 665 A1 is a sensor for the detection of a substance in a liquid. The sensor is designed as a resonator, which is integrated into a measurement chamber by means of a sealing device.
One of the end surfaces of this resonator is in contact with the liquid to be tested. The substance in question, the presence of which is to be detected or the concentration of which is to be measured, accumulates on this end surface and thus leads to a change in the resonance frequency and/or in the vibration amplitude of the resonator. Differences in the properties of the liquid can also cause changes in these two measurement variables.
The invention therefore pertains in particular to the area of piezoelectric quartz oscillator sensors which are used to detect the presence and/or to measure the concentration of biological, chemical, or biochemical substances and of microorganisms in liquids or to determine the properties of a liquid.
With respect to the further functions of a resonator of this type, reference is made to the disclosure content of DE 40 13 665 A1, the entire content of which is to be considered part of the disclosure content of the present invention.
A similar state of the art can be derived from DE 197 34 706 A1, which also describes a piezoelectric resonator, which is integrated into a measurement chamber by means of a sealing arrangement and which is also used to examine biological, chemical, or biochemical substances.
For quartz oscillators operating in the gas phase, quartz oscillator mounts are usually used in which the quartz oscillator is contacted and held in place by two electrical lead wires with bow-like ends. The lead wires pass through an electrically isolated base plate, onto which an electromagnetic shield can be placed. A different type of method for mounting and contacting the oscillator can be found in DE 34 46 612 and DE 199 26 944.
In the known mounting system for piezoelectric quartz oscillators operating in the liquid phase, the upper end surface of the resonator is sealed either by the use of a sealing element, usually a silicone ring, which is pressed onto the upper end surface of the quartz oscillator, or by the use of an adhesive to glue the quartz oscillator into a holder (DE 197 34 706 A1). Contact springs, bond wires, conductive rubber, or conductive adhesive elements are used to establish the necessary electrical contact (DE 401 13 665). In DE 197 34 706 A1, the quartz oscillator is integrated permanently into the measurement chamber, which consists of an injection-molded part, by an adhesive. Electrical contact is accomplished here by bond wires and thus without mechanical tension. This measurement chamber, which is also called a “flow-through cell”, can be installed in, and removed from, the measurement system without affecting the parameters of the quartz oscillator.
In the case of the mounting principles of DE 401 13 665 A1, according to which a sealing ring is pressed onto the upper end surface, it is often true that the surface of the quartz oscillator can be readily accessed for applying a biological coating and that in principle the surface of the quartz oscillator is also optically accessible after it has been installed in the measurement chamber. The use of this sealing principle, however, means that, when the quartz oscillator is removed, it will be subjected to a different degree of mechanical tension when it is reinstalled. This means that the resonator cannot be removed and quickly reinstalled during a measurement to allow the surface of the quartz oscillator to be characterized under a light microscope, a scanning force microscope, etc.
Maintaining uniform electrical contact under this principle is also very difficult or perhaps even impossible. In addition, the process of integrating the quartz oscillator into the measurement chamber or into the measurement system so that the proper contact is achieved is very time-consuming and can be accomplished only by skilled technical personnel.
Not infrequently the pressing of the seal onto the quartz oscillator leads to such strong mechanical stress that the quartz oscillator breaks during installation.
Another disadvantage of this method is that the wetting area of the upper surface of the quartz oscillator is not precisely defined. If the seal is not pressed on firmly enough, wetting liquid escapes, which can cause the two quartz electrodes to become short-circuited. If the quartz oscillator is installed and/or removed with tweezers, for example, it is again possible for the quartz oscillator to be broken, and in cases where a biological coating is present, this coating can be at least partially scraped off or destroyed. A very crucial disadvantage of this mounting principle with respect to the analysis of liquids which contain, for example, “heavy” particles or human cells is the flow barrier necessarily caused by a large seal or a large sealing ring. It is therefore possible only under certain limited conditions to introduce the liquid tangentially, and after a measurement the analyte can be removed, if at all, only by the use of very high flow rates. Such high flow rates affect in turn the oscillation behavior of the quartz oscillator and demand a very tight seal even at high pressures.
In the case of the mounting system described in DE 197 34 706A1, in which the quartz oscillator is integrated permanently into the measurement chamber by an adhesive, it is not possible to characterize the surface of the quartz oscillator by other methods (light microscopy, scanning force microscopy, etc.), nor is it possible to provide a biological coating or to clean the surface of the quartz oscillator outside the measurement chamber. In the case of other mounting principles, according to which the quartz oscillator is glued into a mount, the quartz oscillator can be removed from the measurement chamber as in the case of the “clamping” method, and its surface is therefore often accessible for the application of a biological coating, and in principle it is also usually optically accessible in the installed state.
The systems just mentioned, however, suffer from the disadvantage that the gluing process can subject the quartz oscillator to mechanical stress. Another disadvantage is the laborious nature of the task of gluing the quartz oscillator into the mounting system, which demands that the adhesive be metered and positioned with great accuracy. Residues of adhesive, which result from the imprecise application of the adhesive or from the deposition of adhesive vapors on areas near the center of the oscillator surface, change the oscillation characteristics and often make it impossible to apply a biological coating. Adhesives, such as silicone adhesives, which have the advantage of still having good elasticity values even in the cured state and which are almost completely biocompatible, suffer from the disadvantage of a relatively long curing time. Fast-curing adhesives such as those based on cyanoacrylate have very low elasticity values and as they cure can therefore subject quartz oscillators to severe stress.
A leak-tight seal can be achieved with these adhesives only if the gaps between the mount and the quartz oscillator are very small. This makes assembly extremely difficult. In addition, these adhesives, which have only limited biocompatibility, readily evaporate, and the vapors therefore can easily settle on areas near the center of the resonator.
It is usually quite complicated to produce the necessary electrical contacts in these mounting systems, i.e., the systems in which the quartz resonator is glued in place, because it must be done either simultaneously with the application of the adhesive, or, in the case of the mounts which can be removed from the measurement chamber, by the use of elastic contact pins or the like, which are pressed against the end surface operating in the gas phase.
When contact pins are used to establish electrical contact, the quartz oscillator is again subjected to mechanical stress. When the quartz oscillator is glued in place, there will always be slight differences in positioning between one oscillator and another. This mechanical stress will therefore also be different after the removal and reinstallation of an individual resonator, which makes it difficult to characterize the surface of the oscillator by other methods during a measurement.
As a result of the slight differences in the positioning of the quartz oscillator, the forces which the contact pins exert on the oscillator can also become so strong that the oscillator will break or that the adhesive seal will be loosened and leaks will develop. This problem is often prevented by providing a ring-shaped stop lip, but as a result it becomes difficult if not impossible to introduce a tangential flow in the case of these mounting systems. Because of the disadvantages mentioned above, therefore, it is very difficult to use an adhesive to install quartz oscillators in oscillator mounts in an automated and/or low-cost manner.